To provide wireless service, a wireless carrier typically operates a number of radio access networks, each of which is controlled by a switching entity such as a mobile switching center. The mobile switching center generally includes or connects with one or more base station controllers, which in turn connect with one or more base transceiver stations. Each base transceiver station conventionally includes a cell tower with one or more antennas that radiate to define an air interface in which wireless-communication devices can operate.
To indicate that service is available, a base station sends out a pilot signal that can be picked up by nearby wireless-communication devices. Typically, the pilot signal is a simple, single-frequency signal. To track available base stations, a wireless-communication device maintains an active set of the base stations that are available to the device. Specifically, if the strength of a given pilot signal is above a predetermined threshold, the wireless-communication device may add the base station to the active set.
Wireless-communication devices such as cellular phones, laptops, and personal digital assistants (PDAs) are evolving to provide increasingly complex and data-intensive services to users. As a result, the extent to which users are relying on such wireless-communication devices is increasing rapidly. Users expect their mobile devices to quickly and reliably provide them information in every aspect of their lives.